Catalyst for Removing Nitrogen Oxidase

ABSTRACT

The present invention is to provide a catalyst for removing nitrogen oxides which is capable of keeping sufficient denitrification performance, i.e., a high removal rate of nitrogen oxides in exhaust gas having a high NO 2  content especially under conditions where the ratio of NO 2 /NO in exhaust gas is 1 or higher, a catalyst molded product therefor, and an exhaust gas treating method. The catalyst is designed for removing nitrogen oxides, which is used to denitrify exhaust gas containing nitrogen oxides having a high NO 2  content, which comprises: at least one kind of oxide selected from the group consisting of copper oxides, chromium oxides, and iron oxides as a component for reducing NO 2  to NO; and which further comprises: at least one kind of titanium oxide; at least one kind of tungsten oxide; and at least one kind of vanadium oxide as components for reducing NO to N 2 .

RELATED APPLICATIONS

This application is a continuation application of U.S. application Ser.No. 10/822,441 filed on Apr. 12, 2004, and claims priority from JapaneseApplication No. 2003-113535; filed Apr. 18, 2003 and JapaneseApplication No. 2003-396686; filed Nov. 27, 2003, the disclosures ofwhich are incorporated by reference herein in their entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a catalyst for properly removingnitrogen oxides especially those having a high NO₂ content, a catalystmolded product therefor, and an exhaust gas treating method.

Also, the present invention relates to a combined cycle power generationfacility having an exhaust gas system provided with a catalyst forproperly removing nitrogen oxides especially having a high CO₂ content.

Generally, for nitrogen oxides contained in exhaust gas from a boileretc., NO accounts for 80 to 90% by volume of the nitrogen oxides. As adenitrifying method for such exhaust gas, there is available a method inwhich a catalyst containing, for example, titanium (Ti), tungsten (W),or vanadium (V) as a main active component is used. In this denitrifyingmethod, exhaust gas is caused to pass through the catalyst, and ammoniais added, by which denitrification is accomplished by the followingreaction:

4NO+4NH₃+O₂→4N₂+6H₂O   (i)

However, of various kinds of exhaust gases exhausted recently, exhaustgases from a gas turbine, diesel engine, and gas engine and further achemical plant such as a nitrification plant, which have largefluctuations of load, sometimes have a higher NO₂ content than the NOcontent. Under conditions where the NO₂ content is high, especiallyunder conditions where the ratio of NO₂/NO is 1 or higher, a problem inthat the function of the aforementioned catalyst is insufficient arises.

The catalyst for removing nitrogen oxides in accordance with JapanesePatent Provisional Publication No. 1-151940 (No. 151940/1989) filed bythe applicant for the present invention has been developed as a catalystfor decomposing NO₂. However, this catalyst uses a composite oxide ofcopper and chromium, so that it is difficult to synthesize the oxide andalso it is difficult to demonstrate reproducibility in terms ofperformance.

From the viewpoint of lowering air pollution, as a method for removingnitrogen oxides generated from a boiler and various combustion furnaces,a denitrifying method by catalytic reduction with ammonia, in whichnitrogen oxides are decomposed into nitrogen and water in a contactmanner with a catalyst by using ammonia as a reducing agent, has beenused industrially as the most economical and efficient method. Atpresent, many plants using this method are being operated.

As a catalyst used in this method, a tungsten oxide, vanadium oxide,molybdenum oxide, iron oxide, etc. are carried on a titanium oxide or analuminum oxide. In the treatment of exhaust gas containing sulfuroxides, such as heavy oil or coal burning exhaust gas, a catalyst usinga titanium oxide as a carrier is superior in terms of resistance totoxicity on sulfur oxides. At present, therefore, a titaniumoxide-vanadium oxide-tungsten oxide catalyst is being used as an NOxremoving catalyst (hereinafter referred to as denitrification catalyst)in many actual plants because it is excellent in performance anddurability.

In a boiler, gas turbine, or gas turbine combined cycle for powergeneration, a load fluctuating operation is routinely performedaccording to a change in quantity of generated power. At high load time,there is no hindrance because almost all NOx in exhaust gas exists asNO, but at low load time, almost all NOx in exhaust gas exists as NOz.Therefore, a decrease in NOx removal efficiency of the denitrifier usinga denitrification catalyst may frequently pose a problem of smoke colorcaused by increased concentration of NOx in stack outlet gas.

The inventors of the present invention experimentally analyzed thetitanium oxide-vanadium oxide-tungsten oxide catalyst in a reactionbetween NO and NH₃ expressed by the following formula (1), a reactionbetween NO₂/NO mixing gas and NH₃ expressed by formula (2), and areaction between NO₂ and NH₃ expressed by formula (3).

4NO+4NH₃+O₂→4N₂+6H₂O   (1)

NO+NO₂+2NH₃→2N₂+3H₂O   (2)

6NO₂+8NH₃→7N₂+12H₂O   (3)

As a result, it was revealed that in the case where the mole ratio ofNO₂/NO is 1 or higher or in the case of only NO₂, the NOx removalefficiency of the conventional titanium oxide-vanadium oxide-tungstenoxide catalyst (hereinafter referred to as catalyst A) decreasessignificantly, and it was found that a titanium oxide-vanadiumoxide-tungsten oxide-copper oxide/chromium oxide composite oxide(hereinafter referred to as catalyst B) that takes the place of catalystA is effective (Japanese Patent Provisional Publication No. 1-151940(No. 151940/1989)).

In the gas turbine or gas turbine combined cycle, the NO₂/NO mole ratiovaries relatively widely corresponding to a load fluctuation. At a highload time, almost all NOx is NO and the amount of NO₂ is small. On theother hand, at a low load time the amount of NO₂ increases while theamount of NO is small.

Catalyst A of the conventional composition exhibits a high NOx removalrate when NO concentration is high and NO₂ concentration is low.However, when NO₂ concentration is high and NO concentration is low, theNOx removal rate decreases, so that the NOx removal rate in the case ofhigh NO₂/NO ratio is assumed in designing a denitrifier, so that theamount of catalyst increases, which is economically disadvantageous.

Catalyst B exhibits a high NOx removal rate regardless of variation inthe NO₂/NO ratio. However, catalyst B is more expensive than theconventional catalyst A. Therefore, to replace the conventional catalystA with catalyst B by disposing of catalyst A is economicallydisadvantageous, so that some measures are demanded.

Also, when a denitrifier is already operating smoothly using catalyst Aof the conventional composition in a state in which a decrease incatalyst performance is scarcely recognized and the catalyst can stillbe used continuously, measures for restraining the generation of analarm about increased concentration of exhausted NOx caused by anincrease in NO₂ concentration at low load time are demanded while theexisting denitrification catalyst is made use of to the utmost.

SUMMARY OF THE INVENTION

The present invention has been made to solve the above problems, andaccordingly an object thereof is to provide a catalyst for removingnitrogen oxides which is capable of keeping sufficient denitrificationperformance, i.e., a high removal rate of nitrogen oxides in exhaust gashaving a high NO₂ content especially under conditions where the ratio ofNO₂/NO in exhaust gas is 1 or higher, a catalyst molded producttherefor, and an exhaust gas treating method.

Another object of the present invention is to provide a catalyst forremoving nitrogen oxides which solves a problem with the conventionalmethod for removing nitrogen oxides in exhaust gas and is capable ofalways keeping high NOx removal performance especially without beingaffected by the variations in NO concentration and NO₂ concentration inexhaust gas, a catalyst molded product therefor, and an exhaust gastreating method.

Still another object of the present invention is to provide a combinedcycle power generation facility having an exhaust gas treating systemcapable of keeping a high nitrogen oxides removal rate.

The inventors conducted studies earnestly, and resultantly found outthat if at least one kind of copper (Cu) oxides, chromium (Cr) oxides,and iron (Fe) oxides is contained, sufficient denitrificationperformance can be obtained even for the exhaust gas having a high N—Ocontent.

In accordance with the present invention, there is provided a catalystfor removing nitrogen oxides, which is used to denitrify exhaust gascontaining nitrogen oxides having a high NO₂ content, which comprises:at least one kind of oxide selected from the group consisting of copperoxides, chromium oxides, and iron oxides; and which further comprises:at least one kind of titanium oxide; at least one kind of tungstenoxide; and at least one kind of vanadium oxide.

The present invention provides, as another mode, a catalyst for removingnitrogen oxides, which is used to denitrify exhaust gas containingnitrogen oxides having a high NO₂ content, which comprises: at least onekind of oxide selected from the group consisting of copper oxides,chromium oxides, and iron oxides as a component for reducing NO₂ to NO;and which further comprises: at least one kind of titanium oxide; at onekind of tungsten oxide; and at least one kind of vanadium oxide ascomponents for reducing NO to N₂.

When the component for reducing NO₂ to NO consists of a copper oxide,the catalyst for removing nitrogen oxides contains the oxides so as tocontain 5 to 23 tungsten, 0.1 to 5 vanadium, and 5 or less copper withrespect to 100 titanium in atomic ratio.

When the component for reducing NO₂ to NO consists of a chromium oxide,the catalyst for removing nitrogen oxides contains the oxides so as tocontain 5 to 23 tungsten, 0.1 to 5 vanadium, and 5 or less chromium withrespect to 100 titanium in atomic ratio.

When the component for reducing NO₂ to NO consists of an iron oxide, thecatalyst for removing nitrogen oxides contains the oxides so as tocontain 5 to 23 tungsten, 0.1 to 5 vanadium, and 5 or less iron withrespect to 100 titanium in atomic ratio.

The present invention provides, as another aspect, a catalyst moldedproduct for a catalyst for removing nitrogen oxides, which is obtainedby mixing the component for reducing NO to N₂ with the component forreducing NO₂ to NO.

Such a catalyst molded product can be manufactured by carrying thecomponent for reducing NO₂ to NO on a molded product molded by using thecomponent for reducing NO to N₂. Also, such a catalyst molded productcan be manufactured by carrying the component for reducing NO₂ to NO onthe component for reducing NO to N₂ and then by molding.

The present invention provides, as still another aspect, an exhaust gastreating method using the catalyst for removing nitrogen oxides. In suchan exhaust gas treating method, the NO₂/NO ratio in exhaust gas to betreated is generally 1 or higher. Also, the O₂ concentration in exhaustgas to be treated is generally 6 vol % or higher.

Also, the present invention provides a catalyst for removing nitrogenoxides characterized by comprising a molybdenum oxide, or characterizedby comprising a molybdenum oxide as a component for reducing NO to N₂.

Also, the inventors earnestly carried on studies on a method forremoving nitrogen oxides in exhaust gas, in which high NOx removalperformance is always maintained even if the NO₂ concentration inexhaust gas increases while catalyst A of the conventional composition,which is charged into the existing denitrifier and is being operatedsmoothly in a state in which a decrease in catalyst performance isscarcely recognized, is made use of to the utmost. As a result, it wasfound that a combination of catalyst A of the conventional compositionand catalyst B which achieves high NOx removal performance even if themole ratio of NO₂/NO becomes 1.0 or higher (NO₂ concentration increases)can be applied to a wide range of NOx concentration from NO rich stateat high load to NO₂ rich state at low load. Thereby, the presentinvention has been completed.

A configuration of catalyst for removing nitrogen oxides for achievingthe above object is characterized in that in a catalyst for removingnitrogen oxides which removes nitrogen oxides in exhaust gas byreduction in the presence of ammonia, a first catalyst (catalyst B)which is highly active in removing nitrogen dioxide is arranged on theupstream side in the exhaust gas flow direction, and a second catalyst(catalyst A) which is highly active in removing nitrogen monoxide isarranged on the downstream side of the first catalyst (catalyst B) inthe exhaust gas flow direction.

Another configuration of catalyst for removing nitrogen oxides forachieving the above object is characterized in that in a catalyst forremoving nitrogen oxides which removes nitrogen oxides in exhaust gas byreduction in the presence of ammonia, a first catalyst (catalyst B)which is highly active in removing nitrogen dioxide is arranged on theupstream side in the exhaust gas flow direction, and a second catalyst(catalyst A) which is highly active in removing nitrogen monoxide isarranged on the downstream side of the first catalyst (catalyst B) inthe exhaust gas flow direction; and as the second catalyst (catalyst A),a catalyst comprising a titanium oxide as a first component and at leastone or more kinds of vanadium oxide, tungsten oxide, and molybdenumoxide as a second component is applied, and as the first catalyst(catalyst B), a catalyst in which the second catalyst (catalyst A)comprises at least one or more kinds of copper oxide and chromium oxideas a third component is applied.

Still another configuration of catalyst for removing nitrogen oxides forachieving the above object is characterized in that in a catalyst forremoving nitrogen oxides which removes nitrogen oxides in exhaust gas byreduction in the presence of ammonia, a second catalyst (catalyst A)consisting of a catalyst comprising a titanium oxide as a firstcomponent and a vanadium oxide and a tungsten oxide as secondcomponents, and a first catalyst (catalyst B) consisting of a catalystin which the second catalyst (catalyst A) comprises a composite oxide ofcopper oxide and chromium oxide as a third component are arranged incombination.

Still another configuration of catalyst for removing nitrogen oxides forachieving the above object is characterized in that in a catalyst forremoving nitrogen oxides which removes nitrogen oxides in exhaust gas byreduction in the presence of ammonia, a second catalyst (catalyst A)consisting of a catalyst comprising a titanium oxide as a firstcomponent and a vanadium oxide and a tungsten oxide as secondcomponents, and a first catalyst (catalyst B) consisting of a catalystin which the second catalyst (catalyst A) comprises a composite oxide ofcopper oxide and chromium oxide as a third component are combined; andthe first catalyst (catalyst B) is arranged on the upstream side in theexhaust gas flow direction, and the second catalyst (catalyst A) isarranged on the downstream side of the first catalyst (catalyst B) inthe exhaust gas flow direction.

Also, the catalyst for removing nitrogen oxides is characterized in thatthe catalyst comprises a molybdenum oxide as the second component of thesecond catalyst (catalyst A).

A configuration of catalyst for removing nitrogen oxides for achievingthe above object is characterized in that in a catalyst for removingnitrogen oxides which removes nitrogen oxides in exhaust gas byreduction in the presence of ammonia, a second catalyst (catalyst A)consisting of a catalyst comprising a titanium oxide as a firstcomponent and at least one or more kinds of vanadium oxide, tungstenoxide, and molybdenum oxide as a second component, and a first catalyst(catalyst B) consisting of a catalyst in which the second catalyst(catalyst A) comprises at least one or more kinds of cooper oxide andchromium oxide as a third component are arranged in combination.

Also, the catalyst for removing nitrogen oxides is characterized in thatat least not less than ¼ and less than 4/4 of an upstream catalyst flowpath length in the exhaust gas flow direction is constituted by thesecond catalyst (catalyst A), and a downstream catalyst flow path lengthin the exhaust gas flow direction is constituted by the first catalyst(catalyst B).

The present invention is characterized in that in an exhaust gastreating method for denitrifying exhaust gas containing NO₂ in thepresence of ammonia, NO₂ is reduced to NO by an oxide of at least one ormore kinds of copper oxide and chromium oxide, and NO is reduced to N₂by an oxide of at least one or more kinds of vanadium oxide, tungstenoxide, and molybdenum oxide and a titanium oxide. The present inventionprovides an exhaust gas treating method using the above-describedcatalyst for removing nitrogen oxides.

A configuration of a combined cycle power generation facility forachieving the above object is characterized by comprising a compressorfor compressing air; burning means for burning compressed air compressedby the compressor and a fuel; a gas turbine for generating electricpower by expanding combustion gas sent from the burning means to obtainan output; an exhaust heat recovery boiler in which exhaust from the gasturbine is sent and steam is generated, and the catalyst for removingnitrogen oxides according to any one of claims 17 to 23 is arranged; asteam turbine for generating electric power by expanding steam generatedby the exhaust heat recovery boiler to obtain an output; condensingmeans for condensing exhaust steam of the steam turbine; and supplymeans for supplying condensed water condensed by the condensing means tothe exhaust heat recovery boiler.

As is apparent from the above, according to the present invention, thereare provided a catalyst for removing nitrogen oxides, which is capableof keeping sufficient denitrification performance, i.e., a high removalrate of nitrogen oxides in exhaust gas having a high NO₂ contentespecially under a condition that the ratio of NO₂/NO in exhaust gas is1 or higher, a catalyst molded product therefor, and an exhaust gastreating method.

Tn the catalyst for removing nitrogen oxides and the exhaust gastreating method in accordance with the present invention, catalyst Bexcellent in NO₂ removal performance and catalyst A of the conventionalcomposition excellent in NO removal performance are arranged incombination, and in exhaust gas in which the NO concentration and NO₂concentration vary greatly due to load fluctuations, high NOx removalperformance can be maintained for all loads. Also, when adenitrification catalyst is charged into the existing denitrifier and isbeing operated smoothly in a state in which a decrease in catalystperformance is scarcely recognized, and the catalyst can still be usedcontinuously, the existing denitrification catalyst is taken out, and acomposite oxide of copper and chromium is impregnatingly carriedthereon, subsequently the produced catalyst being recharged. Thereby,the existing denitrification catalyst can be reused, by which effectiveutilization of resources is achieved, and hence a more economicalcatalyst for removing nitrogen oxides and exhaust gas treating methodare provided.

Also, a combined cycle power generation facility is provided in whichcatalyst B excellent in NO₂ removal performance and catalyst A of theconventional composition excellent in NO removal performance arearranged in combination, and in exhaust gas in which the NOconcentration and NO₂ concentration are varied greatly by loadfluctuations, high NOx removal performance can be maintained for allloads.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph showing a change with time of outlet NOx concentrationwhich shows the effect of the nitrogen oxides removing method of thepresent invention; and

FIG. 2 is a schematic view of a combined cycle power generation facilityin accordance with one embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Embodiments of a catalyst for removing nitrogen oxides, a catalystmolded product therefor, and an exhaust gas treating method inaccordance with the present invention will now be described in detail.

The catalyst for removing nitrogen oxides in accordance with the presentinvention is used to denitrify exhaust gas containing nitrogen oxideshaving a high NO₂ content. In particular, this catalyst is suitable fortreating exhaust gas having an NO₂/NO ratio of 1 or higher. Also, thiscatalyst is suitable for treating exhaust gas having an O₂ concentrationof 6 vol % or higher.

The catalyst for removing nitrogen oxides in accordance with the presentinvention contains at least one kind selected from the group consistingof copper oxides, chromium oxides, and iron oxides as a component forreducing NO₂ to NO.

As a cooper source for copper oxides, copper sulfate, copper nitrate,etc. can be cited. As a chromium source for chromium, chromium sulfate,chromium nitrate, chromic acid, ammonium bichronate, etc. can be cited.Also, as an iron source for iron oxides, iron nitrate, iron chloride,iron sulfide, etc. can be cited. From these raw materials, slurry,aqueous solution, etc. for preparing catalyst as in the Examples,described later, can be prepared.

At least one kind of copper oxides, chromium oxides, and iron oxidessuffices. Specifically, the presence of only one kind can achieve asufficiently high removal rate even if the nitrogen oxides have a highNO₂ content. In other words, there is no need for using a compositeoxide, which is difficult to prepare. Conditions permitting, two kindsor all three kinds of these oxides can be contained.

Also, the catalyst for removing nitrogen oxides in accordance with thepresent invention contains at least one kind of titanium oxide, at leastone kind of tungsten oxide, and at least one kind of vanadium oxide ascomponents for reducing NO to N₂.

As a titanium source for titanium oxides, an inorganic titanium compoundsuch as titanium chloride and titanium sulfate, an organic titaniumcompound such as titanium oxalate and tetraalkoxytitanium, etc. can becited. As a vanadium source for vanadium oxides, oxides of vanadium,vanadyl sulfate, vanadyl oxalate, ammonium metavanadate, etc. can becited. As a tungsten source for tungsten oxides, ammonium paratungstate,ammonium metatungstate, etc. can be cited. From these raw materials,slurry, aqueous solution, etc. for preparing catalyst as in theExamples, described later, can be prepared.

Further, in preparing the above-described slurry, aqueous solution, etc.and performing molding, a clay-based inorganic substance such asmontmorillonite, acid clay, bentonite, kaolin, halloysite, and sericite,or an inorganic fiber-form substance such as glass wool, glass fiber,rock wool, and ceramic fiber can be added-to improve the moldability andstrength of catalyst. Also, an organic binder such as cellulose acetatecan also be added. However, from the viewpoint of catalyst activity, itis preferable that the content of a component for these additives withrespect to oxides for achieving the catalyst activity be 30 wt % orlower of the total amount of complete catalyst molded product.

In order to obtain a catalyst molded product by using the catalyst forremoving nitrogen oxides in accordance with the present invention, amethod described below can be used.

As one method, a catalyst molded product can be obtained through aprocess for preparing slurry, aqueous solution, etc. containing acomponent for reducing NO to N₂ and slurry, aqueous solution, etc.containing a component for reducing NO₂ to NO and further through aprocess for mixing these components.

For example, from the aforementioned titanium source, a paste-formtitanium compound such as paste-form titanium hydroxide is obtained.Proper aqueous solutions prepared from the tungsten source and vanadiumsource, for example, methylamine solution of ammonium paratungstate andmethylamine solution of ammonium metavanadate are prepared.Subsequently, these solutions are added in the intended atomic ratio,and are kneaded. The obtained kneaded substance is dried and fired, bywhich a fired body of oxides containing titanium oxides, tungstenoxides, and vanadium oxides is obtained.

The firing operation after drying is generally one mode of evaporationto hardness.

Further, at least one slurry is obtained through a process in which atleast one aqueous solution is prepared from the aforementioned chromiumsource, copper source, and iron source, and the pH is regulated bydripping aqueous ammonia, whereby at least one slurry is obtained.

A crushed substance obtained by crushing the aforementioned fired bodyand at least one of the aforementioned slurries are mixed with eachother and kneaded. This product is molded into, for example, a honeycombshape. After being dried, the molded product is fired to obtain ahoneycomb catalyst. When or after the substance is kneaded, an additivesuch as glass fiber or an organic binder can further be added.

As a molded product, a molded product having such a honeycombconstruction is a typical example. However, the molded product is notlimited to one having this construction.

Also, a catalyst molded product can be manufactured by carrying acomponent for reducing NO₂ to NO on a molded product molded by using acomponent for reducing NO to N₂. What is called a two-layer constructiontype can be molded.

In order to mold a catalyst by using a component for reducing NO to N₂,for example, slurry or aqueous solution containing a component servingas a raw material for catalyst component is prepared and extrusionmolded, and the product molded so as to have a predeterminedconstruction is evaporated to dryness (for example, dried, fired). Inthis case as well, a honeycomb construction obtained by extrusionmolding can be used. For example, from the aforementioned titaniumsource, a paste-form titanium compound such as paste-form titaniumhydroxide is obtained. Proper aqueous solutions prepared from thetungsten source and vanadium source, for example, methylamine solutionof ammonium paratungstate and methylamine solution of ammoniummetavanadate are prepared. Subsequently, these solutions are added inthe intended atomic ratio, and are kneaded. The kneaded substance isextrusion molded, and the product molded so as to have a honeycombconstruction is dried and fired to obtain a honeycomb molded product.When or after the substance is kneaded, an additive such as glass fiberor an organic binder can further be added. Also, an additive such assilica sol can be added to paste-form sodium hydroxide.

In order to carry a component for reducing NO₂ to NO, for example,slurry or aqueous solution containing a component serving as a rawmaterial for catalyst component is prepared, and the aforementionedmolded product is coated (impregnated) with the prepared substance andis dried. For example, at least one aqueous solution is prepared fromthe aforementioned chromium source, copper source, and iron source, andthe molded product is impregnated with this aqueous solution and isdried, by which a catalyst molded product can be obtained.

When a copper oxide is contained as a component for reducing NO₂ to NO,it is preferable that the completed catalyst molded product contain theoxides so as to contain 5 to 23 tungsten, 0.1 to 5 vanadium, and 5 orless copper with respect to 100 titanium in atomic ratio. A lower limitof 0.1 copper with respect to 100 titanium in atomic ratio ispreferable.

When a chromium oxide is contained as a component for reducing NO₂ toNO, it is preferable that the completed catalyst molded product containthe oxides so as to contain 5 to 23 tungsten, 0.1 to 5 vanadium, and 5or less chromium with respect to 100 titanium in atomic ratio. A lowerlimit of 0.1 chromium with respect to 100 titanium in atomic ratio ispreferable.

When an iron oxide is contained as a component for reducing NO₂ to NO,it is preferable that the completed catalyst molded product contain theoxides so as to contain 5 to 23 tungsten, 0.1 to 5 vanadium, and 5 orless iron with respect to 100 titanium in atomic ratio. A content of 0.1to 5 iron with respect to 100 titanium in atomic ratio is preferable.

When all of copper oxide, chromium oxide, and iron oxide are contained,it is preferable that the completed catalyst molded product contain theoxides so as to contain 5 to 23 tungsten, 0.1 to 5 vanadium, and 0.1 to15 total of copper, chromium, and iron with respect to 100 titanium inatomic ratio.

In the exhaust gas treating method in accordance with the presentinvention, he aforementioned copper oxides, chromium oxides, and ironoxides reduce NO₂ to NO according to the following reaction formula:

NO₂→NO+½O₂   (ii)

The ammonia added to the exhaust gas also reduces NO₂ as expressed bythe following formula:

NO₂+2NH₃+O₂→NO+N₂+3H₂O   (iii)

The titanium oxides, tungsten oxides, and vanadium oxides reduce NO toN₂ by the addition of ammonia to exhaust gas as expressed by thefollowing formula:

4NO+4NH₃+O₂→4N₂+6H₂O   (i) (shown before)

The configuration in which the catalyst molded product is molded into ahoneycomb shape and exhaust gas caused to pass through the penetratinghole portion is superior from the viewpoints of construction strengthand denitrification efficiency of catalyst. The temperature of exhaustgas that is caused to pass is preferably 230 to 430° C. The reactiontime is selected from a range of 1000 to 100,000 Nm³/hr per unit cubicmeter of catalyst. The reaction pressure may be atmospheric pressure,reduced pressure, or increased pressure, and is not subject to anyspecial restriction.

As the exhaust gas to be treated, exhaust gases from a gas turbine,diesel engine, and gas engine can be cited as examples.

In order to prepare catalyst A (titanium oxide-vanadium oxide-tungstenoxide), which is the above-described second catalyst, a titanium sourcecan be selected from inorganic titanium compounds such as titaniumchloride and titanium sulfate and organic titanium compounds such astitanium oxalate and tetraalkoxytitanium. A vanadium source can beselected from vanadium oxides, vanadyl sulfate, vanadyl oxalate,ammonium metavanadate, etc. A tungsten source can be selected fromammonium paratungstate, ammonium metatungstate, etc.

Further, to improve the moldability and strength of catalyst, aclay-based inorganic substance such as montmorillonite, acid clay,bentonite, kaoline, halloysite, and sericite, or an inorganic fiber-formsubstance such as glass wool, glass fiber, rock wool, and ceramic fiberis preferably added to the principal component of carrier in an amountof 30 wt %. The catalyst shape may be either a granular shape or ahoneycomb shape. The granular catalyst may be Formed by either thekneading method in which titanium oxide of a carrier component andvanadium oxide-tungsten oxide of a catalyst component are kneaded or theimpregnation method in which a catalyst component is impregnatinglycarried after a carrier is formed.

The honeycomb catalyst may be either of a coat type in which theabove-described carrier component and catalyst component are mixed andslurried to be coatingly carried on a heat resistant honeycomb basematerial such as cordierite, mullite, and silica or of a solid type inwhich the above-described carrier component and catalyst component aremixed homogeneously and extrusion molded into a honeycomb structure.Generally, the weight ratios of vanadium oxide and tungsten oxide withrespect to titanium oxide of 0.001 to 0.1 and 0.01 to 0.2, respectively,are best suitable in terms of economy and performance.

Tn order to prepare catalyst B (titanium oxide-vanadium oxide-tungstenoxide-copper oxide/chromium oxide composite oxide), which is the firstcatalyst, a copper source can be selected from copper sulfate, coppernitrate, etc., and a chromium source can be selected from chromiumsulfate, chromium nitrate, chromic acid, ammonium dichromate, etc.Although, like catalyst A, catalyst B may be granulated and formed bymixing the raw materials, it is generally produced by a method in whicha solution containing a copper source and a chromium source isimpregnatingly carried on the above-described catalyst A. The mixingratio of composite oxide of copper compound and chromium compound withrespect to titanium oxide by oxide-converted weight ratio should be inthe range of 0.002 to 0.2, preferably 0.01 to 0.1. Catalyst B may beprepared newly. However, when the catalyst A having been used for a longperiod of time in a denitrifier in a state in which a decrease incatalyst performance is scarcely recognized and can still he usedcontinuously, the catalyst A is taken out, and a solution containing acopper source and a chromium source is impregnatingly carried on thetaken catalyst A, by which catalyst B can be recharged after beingfired. This method is preferred from the viewpoint of effectiveutilization of resources and economy.

Replacement of all catalysts with catalyst B is unfavorable because ofthe increase in catalyst manufacturing cost and replacing cost. Thearrangement of catalysts A on the downstream side of catalysts B of thepresent invention, which is excellent in NO₂ removal performance, canachieve a sufficient effect in practical use. Although catalysts A andcatalysts B are usually arranged half and half, catalysts B may beincreased or decreased in the range not less than ¼ and less than 4/4according to the increase/decrease in NO₂ concentration (the arrangementof catalyst B only in the range of ¼ on the upstream side can achieve aneffect).

WORKING EXAMPLES Example 1 Preparation of Catalyst Honeycomb Catalyst 1

One thousand five hundred grams of aqueous solution of TiOSO₄ of TiO₂converted concentration 15% was cooled to 20° C. or lower, andneutralized with the pH being 8 by adding 15% aqueous ammonia gradually.The yielded titanium hydroxide precipitate was rinsed and filtered, bywhich paste-form titanium hydroxide 1 was obtained. Methylamine solutionof ammonium paratungstate and ammonium metavanadate was added to thispaste-form titanium hydroxide in a ratio of (Ti:W:V=100:9:3 (atomicratio)), and was kneaded and agitated sufficiently. This kneadedsubstance was fired at 500° C. for five hours after being dried toobtain TiO₂·WO₃·V₂O₅ oxide 1.

Separately, aqueous ammonia was dripped to aqueous solution of ammoniumdichromate ((NH₄)₂Cr₂O₇), copper nitrate (Cu(NO₃)₂·3H₂O), and ironnitrate (Fe(NO₃)₃·6H₂O) and the aqueous solution was agitated, andprecipitate was formed so that the pH was constant, being 7, by whichslurry solution 1 was obtained.

Next, the aforementioned oxide 1 and cake 1 obtained by filtering slurrysolution 1 were kneaded by a kneader, by which kneaded substance 1 wasobtained. The composition of cake 1 in this case was Ti:Cr:Cu:Fe=100:3:2:1 with respect to Ti of oxide 1. Kneading was performed byadding glass fiber, an organic binder (cellulose acetate), and water tokneaded substance 1, and a honeycomb with a pitch of 5 mm and a wallthickness of 1.0 mm was molded by using a honeycomb molding machine. Thecontents of kneaded substance 1, glass fiber, organic binder, and waterwere 100:2:3:25 (by weight) with respect to kneaded substance 1. Afterbeing dried, this honeycomb molded product was fired in air at 500° C.for three hours, by which honeycomb catalyst 1 was obtained.

Honeycomb Catalyst 2 to 9

In the method for preparing the above-described honeycomb catalyst 1,honeycomb catalysts 2 to 7 having the following composition in which theadded amount of chromium, copper, and iron is different were prepared.As oxide 1, the same oxide was used, and as the honeycomb moldingmethod, a method that is the same as the method for honeycomb catalyst 1was used. However, for ammonium dichromate ((NH₄)₂Cr₂O₇), copper nitrate(Cu (NO₃)₂·3H₂O), and iron nitrate (Fe(NO₃)₃·6H₂O), at least one ofthese was used so as to correspond to each prepared honeycomb catalyst.For example, for honeycomb catalyst 9, only iron nitrate was used.

-   -   Honeycomb catalyst 2: Ti:Cr:Cu:Fe=100:4:3:2    -   Honeycomb catalyst 3: Ti:Cr:Cu:Fe=100:3:2:0.3    -   Honeycomb catalyst 4: Ti:Cr:Cu:Fe=100:2:0:1    -   Honeycomb catalyst 5: Ti:Cr:Cu:Fe=100:2:2:0    -   Honeycomb catalyst 6: Ti:Cr:Cu:Fe=100:0:2:2    -   Honeycomb catalyst 7: Ti:Cr:Cu:Fe=100:4:0:0    -   Honeycomb catalyst 8: Ti:Cr:Cu:Fe=100:0:4:0    -   Honeycomb catalyst 9: Ti:Cr:Cu:Fe=100:0:0:3

Honeycomb Catalyst 10 to 12

In the process for preparing slurry solution 1, which has been explainedin preparing the aforementioned honeycomb catalyst 1, aqueous solutionof metal was prepared without the addition of ammonia and withoutyielding precipitates of chromium, copper, and iron. This aqueoussolution was directly carried on crushed substance of oxide 1 by theimpregnation method to yield carrier 1. This carrier 1 was honeycombmolded by the same method as that for honeycomb catalyst 1 to yieldhoneycomb catalyst 10. This honeycomb catalyst 10 has the samecomposition as that of honeycomb catalyst 1. Honeycomb catalysts 11 to12 having different compositions were prepared by the same method. Thecompositions of honeycomb catalysts 10 to 12 are shown below.

-   -   Honeycomb catalyst 10: Ti:Cr:Cu:Fe=100:3:2:1    -   Honeycomb catalyst 11: Ti:Cr:Cu:Fe=100:4:3:2    -   Honeycomb catalyst 12: Ti:Cr:Cu:Fe=100:3:2:0.2

Honeycomb Catalyst 13

When oxide of TiO₂·WO₃·V₂O₅ was prepared by the same method as that forthe aforementioned honeycomb catalyst 1, silica sol (Snowtex-O) wasadded to past-form titanium hydroxide 1 in a ratio of T:Si=100:10(atomic ratio), and TiO₂·SiO₂·WO₃·V₂O₅ oxide was prepared by the samemethod as that for honeycomb catalyst 1. Further, the oxide was kneadedby adding chromium, copper, and iron by the same method as that forhoneycomb catalyst 1, and was honeycomb molded by the same method asthat for honeycomb catalyst 1 to form a catalyst. This catalyst wascalled honeycomb catalyst 13.

Comparative Honeycomb Catalyst 1

In preparing the aforementioned honeycomb catalyst 1, only oxide ofTiO₂·WO₃·V₂O₅ was honeycomb molded without the addition of chromium,copper, and iron by the same method as that for honeycomb catalyst 1 toyield comparative honeycomb catalyst 1.

Example 2 Denitrification Performance Test

Denitrification performance tests were conducted on honeycomb catalysts1 to 13 and comparative honeycomb catalyst 1 obtained in Example 1 underthe conditions described below. The test results (denitrification rate,SO₂ oxidation rate) are given in Table 1.

Catalyst shape: 5 cm×5 cm×100 cm honeycomb shape (volume 2.5 L)Gas amount: 25 Nm³/h (GHS-V 10,000h-1)

Temperature: 280° C., 350° C.

Gas composition (1):NO: 190 ppm, NO₂: 10 ppm, NH₃: 200 ppm, O₂4%, CO₂: 12%, H₂O: 10%, N₂:balanceGas composition (2):NO: 10 ppm, NO₂: 190 ppm, NH₃: 240 ppm, O₂: 4%, CO₂: 12%, H₂O: 10%, N₂:balance

TABLE 1 Catalyst Gas Gas Honeycomb Co-catalyst Composition Compositioncatalyst carrying (1) (2) No. Composition method 280° C. 350° C. 280° C.350° C. 1 Ti, W, V, Kneading 83 92 79 83 Cr, Cu, Fe method 2 Ti, W, V,Kneading 82 89 78 84 Cr, Cu, Fe method 3 Ti, W, V, Kneading 82 90 78 82Cr, Cu, Fe method 4 Ti, W, V, Kneading 81 91 75 82 Cr, Fe method 5 Ti,W, V, Kneading 78 90 75 84 Cr, Cu method 6 Ti, W, V, Kneading 79 90 7283 Cu, Fe method 7 Ti, W, V, Kneading 80 89 73 83 Cr, method 8 Ti, W, V,Kneading 81 89 72 82 Cu, method 9 Ti, W, V, Kneading 80 88 70 81 Femethod 10 Ti, W, V, Impregnatic 79 88 71 82 Cr, Cu, Fe method 11 Ti, W,V, Impregnatic 78 87 70 81 Cr, Cu, Fe method 12 Ti, W, V, Impregnatic 7787 71 83 Cr, Cu, Fe method 13 Ti, Si, W, Kneading 81 88 70 84 V, Cr, Cu,Fe method Comparative 1 Ti, W, V None 83 89 45 61

From the above-described test results, it was confirmed that accordingto the present invention, by adding a co-catalyst (at least one kind ofchromium, copper, and iron) to a basic denitrifying component (titanium,tungsten, vanadium), high denitrification activity is attained evenunder a high NO₂ exhaust gas condition such as gas condition (2).

As is apparent from the above description, according to the presentinvention, there are provided a catalyst for removing nitrogen oxides,which is capable of keeping sufficient denitrification performance,i.e., a high removal rate of nitrogen oxides in exhaust gas having ahigh NO₂ content especially under a condition that the ratio of NO₂/NOin exhaust gas is 1 or higher, a catalyst molded product therefor, andan exhaust gas treating method.

Example 3 Preparation of Catalyst

In a denitrifier of gas turbine combined cycle using liquefied naturalgas as a fuel, a honeycomb catalyst (pitch: 4.2 mm, wall thickness: 0.78mm) having been used for about 94,000 hours was taken as a sample. Thissample was used as catalyst A, being the second catalyst. Catalyst A hada composition of TiO₂:V₂O₆:WO₃=1:0.05:0.08 by weight ratio.

Next, a solution in which chromium nitrate [Cr(NO₃) 9H₂O] and coppernitrate [Cu (No₃)₂·3H₂O] were dissolved in water was prepared. Theconcentration of the solution was adjusted so as to match the waterabsorption coefficient of catalyst A, and the solution was impregnatedand dried, and subsequently was fired at 500° C. for three hours toobtain catalyst B. Catalyst B had a composition ofTiO₂:V₂O₆:WO₃:CuO:Cr₂O₃=1:0.05:0.09:0.0059:0.0112 by weight ratio.Catalyst B can also contain a molybdenum oxide.

Nitrogen Oxides Removal Test

Catalyst A prepared as described above and catalyst B, which is thefirst catalyst, (both catalysts had a cross section of 10 holes×11 holesand a length of 350 mm) were arranged as given in Table 2, and NOxremoval performance tests were conducted under the test conditions givenin Table 3.

First, NH₃ was added so that NH₃/NOx mole ratio was 1.04 with respect tothe inlet NOx concentration (NO: 60 ppm, NO₂: 20 ppm) . After it waschecked that the outlet NOx concentration was stable, the NOconcentration was decreased from 60 ppm to 5 ppm with the NO₂concentration being fixed (20 ppm) to render the inlet NOx concentrationabout 25 ppm. The amount of added NH₃ was decreased simultaneously withthe decrease in NO concentration, and the NO concentration and NH₃concentration were adjusted gradually by taking 15 minutes so that theNH₃ concentration decreased from 83 ppm to 29 ppm. The NH₃/NOx moleratio at this time was 1.16, which corresponded to the NOx removalperformance test assuming low load.

The NOx concentration and NO concentration at the inlet and outlet weremeasured by using a chemiluminescence NOx analyzer The inlet NH₃concentration was determined from the calculated value, and the outletNH₃ concentration was measured by wet analysis.

TABLE 2 Test item NOx NH₃ concentration concentration NH₃/Nox (ppm)(ppm) mole ratio Run Catalyst arrangement NO NO₂ 83→29 1.04→1.16 1→Catalyst A→Catalyst A→Catalyst A→Catalyst A→ 60→5 20 83→29 1.04→1.16 2→Catalyst A→Catalyst A→Catalyst B→Catalyst B→ 60→5 20 83→29 1.04→1.16 3→Catalyst B→Catalyst B→Catalyst A→Catalyst A→ 60→5 20 83→29 1.04→1.16 4→Catalyst B→Catalyst A→Catalyst A→Catalyst A→ 60→5 20 83→29 1.04→1.16 5→Catalyst B→Catalyst B→Catalyst B→Catalyst B→ 60→5 20 83→29 1.04→1.16

TABLE 3 Test Conditions Catalyst shape 42.8 mm × 47.2 mm × 350 mm L ×four Gas amount 20.37 m³N/h Ugs 2.80 mN/sec AV 9.6 m³N/m²h Gastemperature 260° C. Gas composition NOx Given in Table 2 NH₃ Given inTable 2 O₂ 15.8% CO₂ 5% H₂O 5% N₂ Balance

FIG. 1 shows behaviors of outlet NOx concentration and NO concentrationat the time when the inlet NOx concentration was decreased from 80 ppm(NO: 60 ppm, NO_(2B : 20) ppm) at high load to 25 ppm (NO: 5 ppm, NO₂:20 ppm) at low load. Table 4 gives the measurement results of outlet NOxconcentration and NO concentration at high load and outlet NOxconcentration and NO concentration at low load (values after 45minutes).

TABLE 4 Test result High load Low load Nox NO Nox NO concentrationconcentration concentration concentration Run Catalyst arrangement (ppm)(ppm) (ppm) (ppm) 1 Catalyst A-Catalyst A-Catalyst A-Catalyst A 0.2 0.29.6 <0.1 2 Catalyst A-Catalyst A-Catalyst B-Catalyst B 0.6 0.6 7.2 1.9 3Catalyst B-Catalyst B-Catalyst A-Catalyst A 0.3 0.3 0.5 <0.1 4 CatalystB-Catalyst A-Catalyst A-Catalyst A 0.2 0.2 2.4 <0.1 5 CatalystB-Catalyst B-Catalyst B-Catalyst B 0.3 0.3 0.3 <0.1

FIG. 1 shows a change with time of outlet NOx concentration, which showsthe effect of the nitrogen oxides removing method of the presentinvention. In Run 1, only catalysts A were arranged, in Run 2, catalystsB (½ amount) were arranged on the downstream side of catalysts A (½amount), in Run 3, catalysts A (1/2 amount) were arranged on thedownstream side of catalysts B (½ amount), in Run 4, catalysts A (¾amount) were arranged on the downstream side of catalyst 3 (¼ amount),and in Run 5, only catalysts B were arranged.

The following were made apparent from FIG. 1 and Table 4.

At an inlet NOx concentration of 80 ppm (NO: 60 ppm, NO₂: 20 ppm)assuming high load, although the NOx concentration was as high as 80ppm, the outlet NOX concentration decreased to 0.6 ppm or lower whenonly catalysts A of the conventional composition were arranged, whenonly catalysts B excellent in NO₂ removal performance were arranged, orwhen catalyst A and catalyst B were combined, so that an NOx removalrate of 99% or higher was obtained.

On the other hand, at an inlet NOx concentration of 25 ppm (NO: 5 ppm,NO₂: 20 ppm) assuming low load, differences in outlet NOx concentrationwere found, and the outlet NOx concentration decreases in the order ofcatalysts A only, catalysts A (½)+catalysts B (½), catalyst B(¼)+catalysts A (¾), catalysts B (½)+catalysts A (½), and catalysts Bonly.

In particular, a large difference in outlet NOx concentration was founddepending on the combination of catalyst A and catalyst B. Whencatalysts B (½) of the present invention were arranged on the upstreamside, the outlet NOx concentration was 0.5 ppm, so that the NOx removalrate was 98%. On the other hand, when catalysts A (½) were arranged onthe upstream side, the outlet NOx concentration was 7.2 ppm, so that theNOx removal rate was 71%. Thus, a remarkable difference in NOx removalrate was found.

Also, a difference in NOx removal rate was scarcely found between thecase where catalysts B (½) of the present invention were arranged on theupstream side and the case where only catalysts B were arranged.

The above-described catalyst for removing nitrogen oxides, which isarranged by combining catalyst B excellent in NO₂ removal performanceand catalyst A of the conventional composition excellent in NO removalperformance, has the following advantages, and provides a practicalmethod for removing nitrogen oxides in exhaust gas.

(1) in exhaust gas in which the NO concentration and NO₂ concentrationvary greatly from load fluctuations, high NOx removal performance can bemaintained for all loads.(2) When a denitrification catalyst is charged into the existingdenitrifier and is being operated smoothly in a state in which adecrease in catalyst performance is scarcely recognized, and thecatalyst can still be used continuously, the existing denitrificationcatalyst is taken out, and a composite oxide of copper and chromium isimpregnatingly carried thereon, subsequently the produced catalyst beingrecharged. Thereby, the existing denitrification catalyst can be reused,by which effective utilization of resources is achieved, and hence amore economical denitrifying method is provided.

Example 4 Combined Cycle Power Generation Facility

A combined cycle power generation facility provided with theabove-described catalyst for removing nitrogen oxides will be explainedwith reference to FIG. 2. FIG. 2 shows a schematic configuration of acombined cycle power generation facility in accordance with oneembodiment of the present invention.

As shown in FIG. 2, a gas turbine facility 4 which has a compressor 1and a gas turbine 2 and is connected with a generator 3 is provided.Compressed air having been compressed by the compressor 1 is sent to acombustor 5 and is burned together with a fuel. The combustion gas issupplied from the combustor 5 to the gas turbine 2. The gas turbine 2 isdriven by the expansion of combustion gas to obtain an output, by whichelectric power is generated. Exhaust gas having finished work in the gasturbine 2 is discharged into the atmosphere after heat is recovered(steam is generated) by an exhaust heat recovery boiler 6.

The exhaust heat recovery boiler 6 has high-temperature side heatingmeans 11 and low-temperature side heating means 12. In a gas ductbetween the high-temperature side heating means 11 and thelow-temperature side heating means 12, the above-described catalyst forremoving nitrogen oxides (denitrification catalyst) 15 is provided. Thesteam obtained by the high-temperature side heating means 11 is sent toa steam turbine 21 connected with a generator 20, and the steam turbine21 is driven by the expansion of steam to obtain an output, by whichelectric power is generated.

Exhaust steam having finished work in the steam turbine 21 is condensedby a condenser 25. The condensed water condensed by the condenser 25 issupplied to the low-temperature side heating means 12 of the exhaustheat recovery boiler 6 by a feed pump 26.

In the combined cycle power generation facility having the exhaust heatrecovery boiler 6 provided with the denitrification catalyst 15, thedenitrification catalyst 15 in which catalyst B excellent in NO₂ removalperformance and catalyst A of the conventional composition excellent inNO removal performance are combined is provided in an exhaust gastreatment system. Therefore, a combined cycle power generation facilitycan be provided in which high NOx removal performance is maintained forall loads even in exhaust gas under an operating condition that the NOconcentration and NO₂ concentration are varied greatly by loadfluctuations. Also, by charging catalyst B into the existingdenitrifier, or by taking out the existing denitrification catalyst andimpregnatingly carrying a composite oxide of copper and chromiumthereon, and by subsequently recharging the produced catalyst, theexisting denitrification catalyst can be reused, by which effectiveutilization of resources is achieved with a low cost, and hence a moreeconomical combined cycle power generation facility can be provided.

The present invention provides a catalyst for removing nitrogen oxideswhich is capable of keeping sufficient denitrification performance,i.e., a high nitrogen oxides removal rate in exhaust gas having a highNO₂ content especially under a condition that the ratio of NO₂/NO inexhaust gas is 1 or higher, a catalyst molded product therefor, andexhaust gas treating method.

Also, the present invention provides a catalyst for removing nitrogenoxides which solves a problem with the conventional method for removingnitrogen oxides in exhaust gas and is capable of always keeping high NOxremoval performance especially without being affected by the variationsin NO concentration and NO₂ concentration in exhaust gas, a catalystmolded product therefor, and an exhaust gas treating method.

Also, the present invention provides a combined cycle power generationfacility having an exhaust gas treating system capable of keeping a highnitrogen oxides removal rate.

1. A catalyst configuration for removing nitrogen oxides in exhaust gasby reduction in the presence of ammonia, wherein a first catalyst whichis highly active in removing nitrogen dioxide is arranged on theupstream side in the exhaust gas flow direction, and a second catalystwhich is highly active in removing nitrogen monoxide is arranged on thedownstream side of said first catalyst in the exhaust gas flowdirection.
 2. A catalyst configuration for removing nitrogen oxides inexhaust gas by reduction in the presence of ammonia, wherein a firstcatalyst which is highly active in removing nitrogen dioxide is arrangedon the upstream side in the exhaust gas flow direction, said catalystconfiguration comprising: a first catalyst which is highly active inremoving nitrogen dioxide is arranged on the upstream side in theexhaust gas flow direction, said first catalyst comprising at least oneoxide selected from the group consisting of copper oxides and chromiumoxides, and a second catalyst which is active in removing nitrogenmonoxide which is arranged on the downstream side of said first catalystin the exhaust flow direction, said second catalyst comprising atitanium oxide and at least one oxide selected from the group consistingof vanadium oxides, tungsten oxides and molybdenum oxides.
 3. A catalystconfiguration for removing nitrogen oxides in exhaust gas by reductionin the presence of ammonia, said catalyst configuration comprising: afirst catalyst arranged on the upstream side in the exhaust gas flowdirection and comprising a composite oxide of copper oxide and chromiumoxide, and a second catalyst which is active in removing nitrogenmonoxide which is arranged on the downstream side of said first catalystin the exhaust flow direction, said second catalyst comprising atitanium oxide and at least one oxide selected from the group consistingof vanadium oxides, tungsten oxides and molybdenum oxides.
 4. Thecatalyst configuration of claim 3, wherein said second catalystcomprises a titanium oxide, a vanadium oxide and a tungsten oxide. 5.The catalyst configuration of claim 4, wherein said second catalystfurther comprises a molybdenum oxide.
 6. The catalyst configuration ofclaim 1, wherein greater than or equal to ¼ and less than 4/4 of anupstream catalyst flow path length in the exhaust gas flow direction isconstituted by said second catalyst, and a downstream catalyst flow pathlength in the exhaust gas flow direction is constituted by said firstcatalyst.
 7. The catalyst configuration of claim 2, wherein greater thanor equal to ¼ and less than 4/4 of an upstream catalyst flow path lengthin the exhaust gas flow direction is constituted by said secondcatalyst, and a downstream catalyst flow path length in the exhaust gasflow direction is constituted by said first catalyst.
 8. The catalystconfiguration of claim 3, wherein greater than or equal to ¼ and lessthan 4/4 of an upstream catalyst flow path length in the exhaust gasflow direction is constituted by said second catalyst, and a downstreamcatalyst flow path length in the exhaust gas flow direction isconstituted by said first catalyst.
 9. The catalyst configuration ofclaim 4, wherein greater than or equal to ¼ and less than 4/4 of anupstream catalyst flow path length in the exhaust gas flow direction isconstituted by said second catalyst, and a downstream catalyst flow pathlength in the exhaust gas flow direction is constituted by said firstcatalyst.
 10. The catalyst configuration of claim 5, wherein greaterthan or equal to ¼ and less than 4/4 of an upstream catalyst flow pathlength in the exhaust gas flow direction is constituted by said secondcatalyst, and a downstream catalyst flow path length in the exhaust gasflow direction is constituted by said first catalyst.